The 5-HT4 receptor is one of the seven subtypes of 5-Hydroxytryptamine (5-HT) receptors. It is a 7-transmembrane domain protein coupled to a G-protein positively linked to the activation of adenylate cyclase (Molecular Pharmacology, 1990, 37, 408-411). 5-HT4R agonists are found to have potential utility in the treatment of disorders such as Alzheimer's disease, schizpherenia, depression, attention deficit hyperactivity disorder, Huntington's disease, parkinson's disease and several other psychiatric disorders (Behavioral brain research, 1996, 73, 249-252; Neuron, 2007, 55, 712-725; Schizophrenia Bulletin, 2007, 33(5), 1100-1119 and Neuroscience and Medicine, 2011, 2, 87-92). 5-HT4 receptor agonists are known to improve memory in different behavioral experiments in rodents (Neuropharmacology, 1997, 36, 697-706; Journal of Pharmacology and Experimental Therapeutics, 1998, 286, 1115-1121; Journal of Pharmacology and Experimental Therapeutics, 2002, 302, 731-741; Naunyn-Schmiedeberg's Archives of Pharmacology, 2003, 367: 621-628). By injecting 5-HT4 receptor agonist to rats and implanting a recording electrode in the hippocampal CA1 region, Kemp and Manahan-Vaughan (Cerebral Cortex, 2005, 15, 1037-1043) showed that 5-HT4 receptors play a key role in the regulation of synaptic plasticity and the determination of particular properties of stored synaptic information. Autoradiographic studies using the 5-HT4 receptor antagonists [125I]SB207710 and [3H]GR113808 in rat, mouse, guinea pig or post-mortem human brain showed that the 5-HT4 receptor is present at a high density in the limbic system including the hippocampus and frontal cortex (Neuropharmacology 1994, 33, 527-541; European Neuropsychopharmacology, 2003, 13, 228-234) suggesting a role of 5-HT4 receptor in memory and cognition.
The drugs currently available ameliorate late-stage symptoms such as cognitive deficits. No drugs are in the market that specifically targets the cellular mechanisms of Alzheimer's disease (AD), namely the generation of the neurotoxic amyloid β-protein (Aβ) from the amyloid precursor protein (APP). AD is a progressive neurodegenerative disorder characterized by the appearance of senile plaques mainly composed of amyloid β-protein (Aβ), and the development of neurofibrillary tangles in patient's brains (Journal of Neuropathology & Experimental Neurology, 1997, 56, 321-339). AD patients also have cognitive deficits, impaired long-term potentiation (LTP), learning and memory deficits (Neuron, 2004, 44, 181-193) and a consistent deficit in cholinergic neurotransmission. Several acetylcholine esterase inhibitors such as donepezil are available in the market for the treatment of patients with mild-to-moderate AD. However, beneficial effects of this symptomatic treatment can only be maintained for up to 36 months (Pharmacological Research, 2004, 50, 441-451).
Patent publications WO9410174, WO9408994, WO2003035649, WO2004094418, WO2005049608, WO2006090224, WO2011099305, WO2011101774, US20080207690 and US20080269211 disclosed some 5-HT4 receptor compounds. While several 5-HT4 receptor agonists/partial agonists have been disclosed in the literature, no compound, either agonist or partial agonist targeting 5-HT4 receptor is launched in the market until now for the treatment of dementia related disorders. Therefore, there is need and scope to discover new 5-HT4 receptor agonists/partial agonists with novel chemical structures for treatment of disorders that are affected by the 5-HT4 receptor agonists.